As an ink-jet ink for industrial application, an ink which enables printing directly on non-absorptive medium such as a vinyl chloride sheet has been developed in recent years. These inks include a solvent ink which utilizes an organic solvent as a vehicle and a UV ink comprising a polymerizing monomer as a primary component. A solvent ink has a problem of a large amount of VOC, which is a recent social problem, because the solvent is evaporated into the air by drying. Further, there is a risk of influence of such as odor and safety affairs on workers resulting in requirement of a facility countermeasure such as for sufficient ventilation. A UV ink shows VOC of nearly zero since it is cured immediately after printing; however, many of them cause skin sensitization depending on a utilized monomer, which is a problem, and further it is not necessarily applicable in every field because of a restriction from assembling of an expensive UV light source into a printer. Further, in the case of printing a UV ink on a sheet of such as a glossy type, glossy feeling is remarkably deteriorated.
In this background, development of an ink which has small environmental load and enables direct printing on a non-absorptive medium with a water-based ink (a water-based ink-jet recording ink) comprising water as a primary component, which has been conventionally widely utilized in such as home, has been on the way.
In Patent Document 1, a water-based ink containing a solvent compatible with water selected from a group comprising glycol and glycol ether is proposed, and further an ink containing a graft polymer binder which has a hydrophobic, main chain and a nonionic hydrophilic side chain and which is soluble in a water-based vehicle but insoluble in water is proposed. However, it is not disclosed that those having a structure provided with an acid group is utilized as said binder. Further, according to study of inventors of this invention, image quality is not sufficient on a water non-absorptive recording medium according to disclosed technologies, and durability of obtained images is insufficient. In addition to these, recovery through maintenance is not satisfactory. Further, drying property after printing is not sufficient to generate print-through at winding.
In Patent Document 2, a printing ink, for printing on a substrate covered with an untreated vinyl sheet or vinyl, by means of a piezo printing system provided with at least one heating apparatus, which is comprised of a liquid medium, a water-insoluble colorant, a polymer binder, a drying restrainer and other additives, and said liquid medium is constituted of water and a water-miscible drying restrainer, wherein the liquid medium contains at least 80 weight % of water, the content of the drying restrainer is comprised of at least butyl diglycol and 1-methoxy propanol, the ink has a pH value of not less than 8.5, and the binder exists in a state of being dissolved in a liquid ink, has been proposed. According to study of inventors of this invention, in the disclosed technology, image quality on a water non-absorptive recording medium is insufficient, ejection is unstable, and recovery through maintenance is unsatisfactory. Further, drying property after printing is insufficient to generate print-through at winding.
Further, in Patent Documents 3-6, a water-based ink incorporated with resin micro-particles which is insoluble in the ink as a binder resin has been disclosed. This type shows relatively stable ejection behavior, however, image quality on a water non-absorptive medium is insufficient and recovery through maintenance is unsatisfactory.
The following requirements are desired for an ink which is capable of printing also on a water non-absorptive medium.    (1) To be capable of printing on a water non-absorptive recording medium with high quality    (2) Durability of an obtained image is high    (3) Drying on a water non-absorptive medium is rapid    (4) To be capable of stable ejection    (5) To be capable of stable printing for a long period and to exhibit easy recovery through maintenance
However, no inks according to conventional technologies satisfy the all requirements, and argent development is desired.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) JP-A 2000-44858
Patent Document 2: JP-A 2005-113147
Patent Document 3: JP-A 2004-114692
Patent Document 4: JP-A 2005-220352
Patent Document 5: JP-A 2006-22328
Patent Document 6: JP-A 2006-282822